There was recently a discussion on the term “in-law” as in “father-in-law”, “mother-in-law”, “brother-in-law”, “son-in-law” etc.

In looking up something else I came across this entry in Fowler’s Modern English usage, which provides a good summary.

-in-law, describing relationship, was formerly also used in the sense of step- . To Sam Weller [whoever he may be] his father”s second wife was always his mother-in-law; we are not told what he called his own wife’s mother after he married. Today -in-law is never so used; my mother-in-law becomes so by my marriage, my stepmother by hers. The expression in-law derives from the Canon Law prescribing the degrees of affinity within which marriage is prohibited.

The lesson to genealogists is obvious. When you see -in-law, don’t assume what kind of relationship it refers to — always check to make sure.

This happened in our family.

In the 1861 census my ggg grandfather, Simon Hayes, was shown staying in Winscombe, Somerset, with the family of Giles Williams, whose wife was Sidonia.

Simon’s relationship to the head of the household was described as
“brother-in-law”

My first thought (and that of several other researchers into this family) was that Sidonia was Simon’s sister, and that her maiden name was Hayes.

And that was wrong.

After more research I discovered that Sidonia’s maiden name was Sweet.

Simon’s wife was Rachel Allen, and her sister Hester Allen had been Giles
Williams’s first wife, but she died before the 1861 census.

This is not a “step-” relationship, but it is a caution against jumping to
conclusions about the meaning of -in-law.